1. Field of the Invention
The present invention relates to a reception method, reception apparatus and wireless transmission system when adaptive modulation/demodulation is carried out.
2. Description of the Related Art
There are conventional communication systems which select a wireless transmission system according to traffic of each service area, propagation environment and required service content. As conventional wireless communication systems that select this wireless transmission system, there are a low-speed adaptive modulation scheme which takes into consideration traffic, propagation environment, desired transmission speed, etc., when a channel is assigned to each user (communication terminal apparatus) and a high-speed adaptive modulation scheme which adapts its modulation scheme within the assigned range according to instantaneous variations in a propagation path to improve transmission quality.
The low-speed adaptive modulation scheme is basically intended to reduce concentration of traffic of communication systems and at the same time assign transmission bands which will meet requests from users (communication terminal apparatuses) to the greatest possible extent by providing specified quality.
Also, the high-speed adaptive modulation scheme is intended to monitor instantaneous C/Ic (power ratio of a desired signal to an interference signal: carrier to co-channel power ratio) and instantaneous delay spread and at the same time realize high throughput with high transmission quality by selecting an optimum number of modulation multi-values and symbol rate within the assigned band.
Communication systems using these schemes are easily adaptable to implementation of a large-capacity system, buffering of load on communication systems in response to dynamic variations of traffic and handling of services for media with different transmission speeds.
Meanwhile, as modulation schemes that can be switched in a communication system, there are QPSK (Quaternary Phase Shift Keying), 16QAM (16 Quadrature Amplitude Modulation) and 64QAM, etc. In these modulation schemes, frequency utilization efficiency increases from QPSK, 16QAM and 64QAM in that order, but with the increase of frequency utilization efficiency, an inter-signal distance also decreases in that order, which makes the system vulnerable to noise.
Therefore, an adaptive modulation scheme that adaptively switches between these modulation schemes, selects, for example, a modulation index (number of modulation multi-values) based on its average CNR, and can thereby select an optimum modulation scheme according to its propagation path environment (e.g., see the Unexamined Japanese Patent Publication No. HEI 11-275164, p 6, FIG. 2).
FIG. 1 is a block diagram showing a configuration of a conventional reception apparatus of a system that demodulates waveform distortion due to fading through adaptive equalization processing. Here, a TDMA (Time Division Multiple Access)-based communication system will be shown and suppose that known signal patterns (training signals) used for synchronization acquisition and transmission path estimation are inserted in processing units (burst, etc.) of a received signal. However, suppose information on a modulation index is not included in the received signal.
In FIG. 1, a reception apparatus 1 performs reception processing on the received signal at a wireless section 2 and then supplies this to an A/D (analog/digital) conversion section 3. The analog/digital conversion section 3 converts the received signal supplied from the wireless section 2 to a digital signal and then supplies it to a reception level measuring section 4, a transmission path estimation section 5, a synchronization section 6 and a demodulation scheme estimation decision section 10.
The reception level measuring section 4 measures intensity of the digital received signal (RSSI (Received Signal Strength Indicator) signal level) supplied from the A/D conversion section 3 and thereby performs gain control on the wireless section 2. The transmission path estimation section 5 performs transmission path estimation based on the digital received signal supplied from the A/D conversion section 3 and then supplies the result to a switching section 7. Furthermore, the synchronization section 6 performs coherent detection based on the digital received signal to acquire synchronization with the transmission path estimation section 5 and demodulation scheme estimation decision section 10.
The demodulation scheme estimation decision section 10 supplies the digital received signal supplied from the A/D conversion section 3 to demodulation estimation sections 11, 12 and 13. The demodulation estimation sections 11, 12 and 13 each demodulate a portion of the received signal (which is a training signal in a burst and portion modulated in the same way as for transmission data (data channel)) according to different demodulation schemes. For example, the demodulation estimation section 11 QPSK-demodulates the received signal, the demodulation estimation section 12 16QAM-demodulates the received signal and the demodulation estimation section 13 64QAM-demodulates the received signal.
Then, the demodulation estimation section 11 supplies the result of demodulation according to the assigned demodulation scheme to a likelihood calculation section 14, the demodulation estimation section 12 supplies the result of demodulation according to the assigned demodulation scheme to a likelihood calculation section 15 and the demodulation estimation section 13 supplies the result of demodulation according to the assigned demodulation scheme to a likelihood calculation section 16.
The likelihood calculation section 14 uses a training signal supplied from a training signal generation section 17 as an expected value and calculates a mean square error between the expected value and received signal as likelihood. On the other hand, the likelihood calculation section 15 uses a training signal supplied from a training signal generation section 18 as an expected value and calculates a mean square error between the expected value and received signal as likelihood. Furthermore, the likelihood calculation section 16 uses a training signal supplied from a training signal generation section 19 as an expected value and calculates a mean square error between the expected value and received signal as likelihood.
Training signals supplied from the training signal generation sections 17, 18 and 19 are the same training signals included in their respective received signals and the likelihood calculation sections 14, 15 and 16 calculate likelihood values using these training signals as expected values, and can thereby estimate which demodulation scheme is the most likely one, that is, estimate the demodulation schemes of the received signals according to which modulation was carried out on the transmitting side. This estimation processing is carried out by a demodulation scheme decision section 20 which has received the likelihood calculation results from the likelihood calculation sections 14, 15 and 16.
The result decided by the demodulation scheme decision section 20 is supplied to the switching sections 7 and 24 as a demodulation scheme selection signal. The switching section 7 selects any one of a plurality of equalization/demodulation sections 21, 22 and 23 provided beforehand according to expected demodulation schemes based on the demodulation scheme selection signal supplied from the demodulation scheme decision section 20.
That is, the equalization/demodulation section 21 is designed to demodulate the received signal using the same demodulation scheme as that used for demodulation at the demodulation estimation section 11, the equalization/demodulation section 22 is designed to demodulate the received signal using the same demodulation scheme as that used for demodulation at the demodulation estimation section 12 and the equalization/demodulation section 23 is designed to demodulate the received signal using the same demodulation scheme as that used for demodulation at the demodulation estimation section 13.
Therefore, the demodulation scheme decision section 20 selects the equalization/demodulation section capable of obtaining the most likely demodulation result based on the result of likelihood calculations from among the equalization/demodulation sections 21, 22 and 23 based on the demodulated signals supplied from the demodulation estimation sections 11, 12 and 13, and can thereby select the demodulation scheme corresponding to the demodulation scheme estimated to be the same as the modulation scheme on the transmitting apparatus side and demodulate the received signal.
The signals demodulated by the equalization/demodulation sections 21, 22 and 23 are supplied to an error correction section 25 through a switching section 24, subjected to forward error correction processing and then supplied to a reception quality measuring section 26. The reception quality measuring section 26 measures reception quality of decoded data supplied from the error correction section 25.
Thus, the reception apparatus 1 shown in FIG. 1 provides a demodulation section (demodulation estimation sections 11, 12 and 13) for each of a plurality of demodulation schemes corresponding to a plurality of demodulation schemes expected as the modulation scheme on the transmitting apparatus side, estimates the modulation scheme on the transmitting apparatus side based on the respective results of demodulation processing on the received signal through these demodulation sections, carries out demodulation processing according to this estimation result, and can thereby estimate the modulation scheme on the transmitting apparatus side through the reception apparatus 1 without sending any signal indicating the modulation scheme from the transmitting apparatus side.
However, the conventional reception apparatus 1 has a problem that as a method of estimating the demodulation scheme of a received signal, providing a plurality of demodulation sections (demodulation estimation sections 11, 12 and 13) corresponding to the estimated demodulation schemes for the demodulation scheme estimation decision section 10 and carrying out demodulation processing corresponding to the expected demodulation schemes at the demodulation estimation sections 11, 12 and 13 will increase the amount of processing and also increase power consumption.